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Immobilized metal ion affinity chromatography (IMAC) is an important chromatographic technique for biomolecules. In order to get a detailed understanding of the hydration of immobilized Fe(III), complexes of Fe(III) with methyl substituted iminodiacetate ([Fe(MSIDA)(H2O)3](+)) as well as with methyl substituted nitrilotriacetate ([Fe(MSNTA)(H2O)2]) were simulated in aqueous solutions with the quantum mechanical charge field molecular dynamics (QMCF MD) approach. The simulations were carried out at the Hartree-Fock (HF) level of theory, since cluster calculations at the HF, MP2, and B3LYP levels of theory showed that this method results in a good compromise between computational effort and accuracy. None of the coordinating water molecules were exchanged during the simulation period of 15 ps. The Fe-OH2O bond distances as well as the Fe-OH2O stretching motions differed among the coordinating water molecules, indicating different bond strengths. For the water molecules in the second hydration layer, mean residence times of 2.7 and 1.9 ps were obtained for [Fe(MSIDA)(H2O)3](+) and [Fe(MSNTA)(H2O)2], respectively. Furthermore, infrared measurements were carried out to characterize the most prominent bond features of aqueous Fe(III)-NTA and to discuss these results in conjunction with the computationally derived frequencies.

The cis- and trans-bis(glycinato)copper(II) complexes in aqueous solution have been investigated by means of a combined theoretical and experimental approach. The conducted quantum mechanical charge field molecular dynamics (QMCF-MD) studies, being the first quantum mechanical simulations of organometallic complexes by this method, yielded accurate structural details of the investigated isomers as well as novel dynamic data, which has successfully been confirmed and extended by subsequent mid-infrared measurements. The spectroscopic results, critically assessed by adjacent multivariate data analysis, indicate an isomeric stability at ambient conditions, vanishing at elevated temperatures.

The aqueous solutions of bisulfite (SO(3)H(-)) and sulfonate (HSO(3)(-)) were simulated by the ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism. All superimposed trajectories for the atomic coordinates of solutes with three-dimensional alignment here illustrated the reactivities of the ions. Power spectra were evaluated on the basis of the velocity autocorrelation functions (VACFs) with the normal-mode analysis, presenting a higher frequency of the symmetric SO(3) deformation (δ(s)(SO(3))) than the asymmetric SO(3) deformation (δ(as)(SO(3))) modes for the sulfonate ion. The different influence of solvent on the frequency of the O-H and S-H stretching suggests a higher stability of hydrated sulfonate ion. The bisulfite shows a slightly stronger molecular hydration shell than the sulfonate ion with the average number of ion-solvent hydrogen bonds (H-bonds) of 5.3 and 5.0, respectively. Extra water molecules within the molecular hydration shell are found for bisulfite (1.2) and for sulfonate (1.6). The mean residence times for the water ligands classify each ion as a structure maker, while the S-H bond within the sulfonate ion displays a hydrophobic behavior. No tautomerization was observed within the simulation period.

The hydration of the Bi(III) ion was determined via an ab initio quantum mechanical charge field molecular dynamics (QMCF-MD) simulation. Ten picosecond sampling was carried out to determine structural and dynamical properties of the Bi(III) ion in aqueous solution. In the first hydration shell, the ion is 9-fold coordinated with a maximum probability of the Bi-O distance at 2.51 Å. In total, 11 exchanges were observed in the first-shell showing associative, dissociative, and interexchange character. As with the dominant existence of 9-fold coordination, the geometry of the Bi(III) ion is in between the tricapped trigonal prism and the capped square antiprism.

An ab initio Quantum Mechanical Charge Field Molecular Dynamics Simulation (QMCF MD) was performed to investigate structure and dynamics behavior of hydrated sulfur dioxide (SO(2)) at the Hartree-Fock level of theory employing Dunning DZP basis sets for solute and solvent molecules. The intramolecular structural characteristics of SO(2), such as S═O bond lengths and O═S═O bond angle, are in good agreement with the data available from a number of different experiments. The structural features of the hydrated SO(2) were primarily evaluated in the form of S-O(wat) and O(SO(2))-H(wat) radial distribution functions (RDFs) which gave mean distances of 2.9 and 2.2 Å, respectively. The dynamical behavior characterizes the solute molecule to have structure making properties in aqueous solution or water aerosols, where the hydrated SO(2) can easily get oxidized to form a number of sulfur(VI) species, which are believed to play an important role in the atmospheric processes.

The ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism was applied to simulate the bisulfate ion, HSO4-, in aqueous solution. The averaged geometry of bisulfate ion supports the separation of six normal modes of the O*-SO3 unit with C3v symmetry from three modes of the OH group in the evaluation of vibrational spectra obtained from the velocity autocorrelation functions (VACFs) with subsequent normal coordinate analyses. The calculated frequencies are in good agreement with the observations in Raman and IR experiments. The difference of the averaged coordination number obtained for the whole molecule (8.0) and the summation over coordinating sites (10.9) indicates some water molecules to be located in the overlapping volumes of individual hydration spheres. The averaged number of hydrogen bonds (H-bonds) during the simulation period (5.8) indicates that some water molecules are situated in the molecular hydration shell with an unsuitable orientation to form a hydrogen bond with the ion. The mean residence time in the surroundings of the bisulfate ion classify it generally as a weak structure-making ion, but the analysis of the individual sites reveals a more complex behavior of them, in particular a strong interaction with a water molecule at the hydrogen site.

Ab initio quantum mechanical charge field molecular dynamics (QMCF-MD) simulations have been performed for aqueous solutions of Fe(2+) and Fe(3+) ions at the Hartree-Fock level of theory to describe and compare their structural and dynamical behavior. The structural features of both hydrated ions are characterized by radial distribution functions that give the maximum probability of the ion-O distance for Fe(2+) and Fe(3+) ions at 2.15 and 2.03 A, respectively. The angular distribution functions of both ions prove the octahedral arrangement of six water ligands, whereas the second shells of these ions differ. Both ions show influence on the water molecules beyond the second shells. The structure-forming abilities of both ions are visible from the ligand mean residence times and ion-O stretching frequencies evaluated for both ions. The substantially improved data obtained from these QMCF-MD simulations show better correlation with available experimental results than the conventional quantum mechanics/molecular mechanics molecular dynamics (QM/MM MD) approaches with one hydration shell treated by quantum mechanics.

The structure and dynamics of the stable four-times positively charged uranium(IV) cation in aqueous solution have been investigated by ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) simulation at the Hartree-Fock double-zeta quantum mechanical level. The QMCF-MD approach enables investigations with the accuracy of a quantum mechanics/molecular mechanics approach without the need for the construction of solute-solvent potentials. Angular distribution functions; radial distribution functions; coordination numbers of the first, second, and third shell (9, 19, and 44, respectively); coordination number distribution functions; tilt- and Theta-angle distribution functions; as well as local density corrected triangle distribution functions have been employed for the evaluation of the hydrated ion's structure. Special attention was paid to the determination of the geometry of the first hydration layer, and the results were compared to experimental large-angle X-ray scattering and extended X-ray absorption fine structure data. The solvent dynamics around the ion were also investigated using mean ligand residence times and related data and, resulting from the unavailability of any experimental data, were compared to ions with similar properties.

Quantum mechanical/molecular mechanical (QM/MM) and quantum mechanical charge field (QMCF) molecular (MD) simulations have been performed to describe structural and dynamical properties of Na(I) and K(I) in water and to compare the two approaches. The first and second hydration shells were treated by ab initio quantum mechanics at the restricted Hartree-Fock (RHF) level. The structural data are in good agreement with previously published experimental and theoretical results. A considerable number of water exchange reactions were observed within the first shell during the simulation time of 12 ps. The number of exchange events in both shells is higher in the case of K(I) than Na(I) reflecting the weaker ion-ligand bond strength of K(I). Comparison of the "conventional" QM/MM framework with the QMCF method clearly indicates the latter to be advantageous, as ambiguities arising from the coupling of the subregions occurring in the QM/MM MD simulations did not evolve when the QMCF ansatz was applied.

The structural and dynamical properties of the cis-(O-Zn-O angle approximately 90 degrees) and trans-(O-Zn-O angle approximately 180 degrees) isomers of the model diaquotriamminezinc(II) complex in aqueous solution have been evaluated using the hybrid quantum mechanical/molecular mechanical molecular dynamics simulation approach at ab initio Hartree-Fock level. In both complexes, the first hydration shell contains five ligands (two water and three ammonia molecules) arranged in a trigonal bipyramidal geometry. In the metastable cis-isomer two different bond lengths of 2.34 and 2.13 A are observed for the Zn-Oax and Zn-Oeq bonds, respectively. The trans-isomer shows the maximum of the Zn-O distance at 2.26 A. The Zn-N bond distances in both cases are approximately 2.12 A. A geometrical transformation of the cis-isomer into the trans-isomer was observed after 11.5 ps of simulation, and the trans-isomer then remained stable throughout the whole simulation time of 30 ps. A comparative study for both isomers has been performed in terms of radial distribution functions, coordination number distributions, angular distribution functions, tilt and theta angle distributions, ligands' mean residence time, ion-ligand stretching frequencies, and the vibrational and librational motions of water ligands. The results are compared with the data for the previously studied zinc-monoamine and -diamine complexes.

The hydrated sulfate ion has been characterized in aqueous solution in structural and dynamic aspects using ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) simulation and large angle X-ray scattering (LAXS) methods. The LAXS data show an average coordination number of the sulfate ion of up to 12 water molecules bound through hydrogen bonding, while the QMCF MD simulation displays a wide range of coordination numbers between 8 and 14 with an average value of approximately 11. The Os...Ow distance cannot be distinguished from the Ow...Ow distance in the LAXS experiment; the weighted mean O...O distance is 2.880(10) A. In the simulation, the Os...Ow and Ow...Ow distances are found to be very similar, namely, 2.86 and 2.84 A, respectively. The S-Os bond and S...Ow distance have been determined by the LAXS experiment as 1.495(6) and 3.61(2) A, respectively, indicating an average nearly tetrahedral S-Os...Ow angle. The approximately 5% deviations of simulation distances (1.47 and 3.82 A) from the experimental ones can probably be ascribed to the neglect of correlation energy in the quantum mechanical method. The mean residence time of water ligands at O atoms, 2.57 ps, is longer than that in pure water, 1.7 ps, characterizing the sulfate ion as a weak structure maker.

The structural and dynamical properties of NO3- in dilute aqueous solution have been investigated by means of two combined quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations, namely HF/MM and B3LYP/MM, in which the ion and its surrounding water molecules were treated at HF and B3LYP levels of accuracy, respectively, using the DZV+ basis set. On the basis of both HF and B3LYP methods, a well-defined first hydration shell of NO3- is obtainable, but the shell is quite flexible and the hydrogen-bond interactions between NO3- and water are rather weak. With respect to the detailed analysis of the geometrical arrangement and vibrations of NO3-, the experimentally observed solvent-induced symmetry breaking of the ion is well reflected. In addition, the dynamical information, i.e., the bond distortions and shifts in the corresponding bending and stretching frequencies as well as the mean residence time of water molecules surrounding the NO3- ion, clearly indicates the "structure-breaking" ability of this ion in aqueous solution. From a methodical point of view it seems that both the HF and B3LYP methods are not too different in describing this hydrated ion by means of a QM/MM simulation. However, the detailed analysis of the dynamics properties indicates a better suitability of the HF method compared to the B3LYP-DFT approach.

Interactions between metal ions and amino acids are common both in solution and in the gas phase. Here, the effect of metal ions and water on the structure of glycine is examined. The effect of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+) and water on structures of Gly.Mn+(H2O)m and GlyZwitt.Mn+(H2O)m (m = 0, 2, 5) complexes have been determined theoretically by employing the hybrid B3LYP exchange-correlation functional and using extended basis sets. Selected calculations were carried out also by means of CBS-QB3 model chemistry. The interaction enthalpies, entropies, and Gibbs energies of eight complexes Gly.Mn+ (Mn+ = Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+) were determined at the B3LYP density functional level of theory. The computed Gibbs energies DeltaG degrees are negative and span a rather broad energy interval (from -90 to -1100 kJ mol(-1)), meaning that the ions studied form strong complexes. The largest interaction Gibbs energy (-1076 kJ mol(-1)) was computed for the NiGly2+ complex. Calculations of the molecular structure and relative stability of the Gly.Mn+(H2O)m and GlyZwitt.Mn+(H2O)m (Mn+ = Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+; m = 0, 2, and 5) systems indicate that in the complexes with monovalent metal cations the most stable species are the NO coordinated metal cations in non-zwitterionic glycine. Divalent cations Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+ prefer coordination via the OO bifurcated bonds of the zwitterionic glycine. Stepwise addition of two and five water molecules leads to considerable changes in the relative stability of the hydrated species. Addition of two water molecules at the metal ion in both Gly.Mn+ and GlyZwitt.Mn+ complexes reduces the relative stability of metallic complexes of glycine. For Mn+ = Li+ or Na+, the addition of five water molecules does not change the relative order of stability. In the Gly.K+ complex, the solvation shell of water molecules around K+ ion has, because of the larger size of the potassium cation, a different structure with a reduced number of hydrogen-bonded contacts. This results in a net preference (by 10.3 kJ mol(-1)) of the GlyZwitt.K+H2O5 system. Addition of five water molecules to the glycine complexes containing divalent cations Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+ results in a net preference for non-zwitterionic glycine species. The computed relative Gibbs energies are quite high (-10 to -38 kJ mol(-1)), and the NO coordination is preferred in the Gly.Mn+(H2O)5 (Mn+ = Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+) complexes over the OO coordination.

Structure and dynamics investigations of Ag(+) in 18.6% aqueous ammonia solution have been carried out by means of the ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulation method. The most important region, the first solvation shell, was treated by ab initio quantum mechanics at the Restricted Hartree-Fock (RHF) level using double-zeta plus polarization basis sets for ammonia and plus ECP for Ag(+). For the remaining region in the system, newly constructed three-body corrected potential functions were used. The average composition of the first solvation shell was found to be [Ag(NH(3))(2)(H(2)O)(2.8)](+). No ammonia exchange process was observed for the first solvation shell, whereas ligand exchange processes occurred with a very short mean residence time of 1.1 ps for the water ligands. No distinct second solvation shell was observed in this simulation.

Structural and dynamical properties of the hydrated Cs+ ion have been investigated by performing ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations at different quantum mechanical levels (HF, B3LYP and BP86). The first shell coordination number was found to be ∼8 in the HF and ∼9 in the B3LYP and BP86 case and several other structural parameters such as angular distribution functions, radial distribution functions, and tilt- and θ-angle distributions allowed to fully characterize the hydration structure of the Cs+ ion. Velocity autocorrelation functions were used to calculate librational and vibrational motions, ion−ligand motions, as well as reorientation times. The strong “structure breaking” effect of Cs+ can be interpreted on the basis of different dynamical parameters such as accelerated water reorientation, mean ligand residence time, and the number of ligand exchange processes.

A hybrid ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulation has been performed to evaluate hydration structure and dynamics of the Cd 2 + ion in aqueous solution. Two hydration shellswere identified at the mean distances of 2.3 and 4.8 A, with coordination numbers of 6 and ∼12, respectively, which is in good agreement with X-ray data. Librational and vibrational spectra of the hydrated Cd 2 + ion have been evaluated, showing a fairly complex spectrum for Cd-O vibrations, whose stretching force constant of 68 N m - 1 is slightly higher than that for Hg-O vibrations. The mean residence time for the 2nd shell ligands was determined as 10 ps, and the calculated hydration energy of -458 kcal/ mol is close to the experimental value of -438 kcal/mol.

Alkaloids isolated from Sarcococca saligna significantly inhibit acetyl- and butyrylcholinesterase enzyme, suggesting discovery of inhibitors for nervous-system disorders. Studying interactions with the active site of the AChE enzyme from Torpedo californica, we have identified hydrophobic interactions inside the aromatic gorge area as the major stabilizing factor in enzyme-inhibitor complexes of these alkaloids. Molecular Dynamics simulation of a predicted complex indicates that ligand binding does not extensively alter enzyme structure, but reduces flexibility at the gorge.

A molecular dynamics simulation based on ab initio quantum mechanical forces in combination with molecular mechanics has been performed to describe structural and dynamical properties of Ag+ in water. The first hydration shell, being the chemically most relevant region, was treated by quantum mechanics at Hartree−Fock level using the LANL2DZ ECP for Ag+ and double-ζ plus polarization basis sets for water. The outer region of the system was described using a newly constructed classical three-body corrected potential derived from ab initio energy surfaces. The structure was evaluated in terms of radial and angular distribution functions and coordination number distributions. Water exchange processes between coordination shells have been investigated and evaluated. The results show that the first hydration shell is of rather irregular shape, with an average coordination number of 5.5. Fast water exchange processes between the first and second hydration shell were observed, leading to a preference of 5- and 6...

The hydration shell structure of Fe(II) and Fe(III) ions in their high spin state has been studied by combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations, in which the ion and its first hydration sphere were treated at the Hartree−Fock ab initio quantum mechanical level, whereas ab initio generated pair plus three-body potentials were employed for the remaining system. The coordination number in the first hydration shell is 6 for both Fe(II) and Fe(III) ions. The second hydration shell contains 12.4 and 13.4 water molecules for Fe(II) and Fe(III) ions, respectively, in good agreement with the experimental values. The residence time of a water molecule in the second hydration shells of Fe(II) and Fe(III) is 24 and 48 ps, respectively. The complex configuration and ligand orientations observed in this study prove that many-body effects play an important role in the hydration of both Fe(II) and Fe(III) ions. The hydration energies computed from the simulation are...

Molecular dynamics (MD) simulations based on ab initio evaluated two- and three-body potentials were performed for Ti3+, Cr3+, and Co3+ ions in water. The ions' hydration structure was evaluated in terms of radial distribution functions, coordination numbers, and angular distributions. The first solvation shell shows an exact coordination number of 6 for all ions and average M3+−O distances of 2.08, 2.05, and 2.03 A for Ti3+, Cr3+, and Co3+, respectively. The structural parameters obtained after inclusion of three-body correction terms are in good agreement with experimental values. The energies of hydration obtained from three-body corrected molecular dynamics simulations are also in good agreement with experiment, except for Ti3+ where the classical simulations are failing to reproduce the Jahn−Teller effect.

Two different quantum mechanics/molecular mechanics molecular dynamics simulation techniques (QM/MM-MD) are compared using as an example the hydrated manganous ion. One includes a perturbation field composed of point charges representing the bulk solvent molecules polarizing the QM region in addition to a quantum chemically derived two-body potential. The other includes the two-body potential plus a three-body correction function. In both simulations, the QM region comprises the ion and its first hydration shell and hence includes all many-body terms up to the QM frontier. The structure of the hydrated ion is discussed in terms of radial distribution functions, coordination numbers, and angular distributions. The results show that both QM/MM-MD techniques agree well with experimental findings for the first-shell hydration structure. QM/MM-MD including point charges has been found to be a satisfactory and economic alternative in studying the first hydration shell without the need to undergo the time-consum...

Classical molecular dynamics (MD), classical Monte Carlo (MC), and combined quantum mechanical/molecular mechanical (QM/MM) MD simulations were carried out to investigate the hydration structure of the Ni(II) ion in water using a newly constructed 2-body potential and a function correcting for 3-body effects. A 6-coordinate hydration structure with a maximum probability of the Ni−O distance at 2.25, 2.21, and 2.14 A was observed by the classical MD, classical MC, and QM/MM-MD simulation with 3-body corrections, respectively, while an 8-coordinate structure was observed by the classical MD and MC simulations using only 2-body pair potentials. The average structure parameters obtained by the Hertree-Fock level QM/MM-MD simulation are in agreement with the experimental values. The validity of the 3-body correction function is discussed on the basis of the results for the classical and QM/MM simulations. During the classical MD simulation, a water exchange reaction was observed for the 6-coordinate Ni(II) ion...

An ab intio two-body analytical potential function was constructed to describe Mn(II)−water interactions. Classical Monte Carlo (MC) and molecular dynamics (MD) simulations have been performed to s...

An ab initio molecular dynamics simulation based on combined quantum mechanical and molecular mechanical (QM/MM) potential was performed for Na+ in liquid ammonia. With this approach, many-body interactions that are very significant in determining the solvation number are modeled accurately by quantum mechanics. The method appears to resolve the discrepancy between many simulations that have predicted solvation numbers between 5 and 9. Although the solvation number of 5 obtained from our QM/MM simulation has not yet been confirmed by the experimental data, a detailed analysis of the simulation results together with a comparison of previous studies on known systems justifies this prediction.

The Cu2+ hydration shell structure has been studied by a combined ab initio quantum mechanical/molecular mechanical (QM/MM) Monte Carlo simulation, in which the ion and its first hydration sphere are treated at the Born−Oppenheimer ab initio quantum mechanical level, while classical pair and three-body potentials are employed for the remaining system. Whereas traditional simulations based on MM potentials are not able to predict higher-body effects such as the Jahn−Teller (JT) distorted octahedral structure of the first hydration shell of Cu2+, the combined QM/MM approach reproduces correctly this experimentally confirmed property and delivers more accurate hydration energy values as well.

The solvation structure of Cu+ in water and in liquid ammonia has been investigated using the Metropolis Monte Carlo method. The systems consisting of one Cu+ in 215 solvent molecules have been simulated at a temperature of 240 K for ammonia and 298 K for water, respectively. Cu+−ammonia and Cu+−water pair potentials have been newly developed based on ab initio calculations of double-ζ quality. Structural properties were investigated by means of radial distribution functions and their running integration numbers, leading for the first solvation shell to an average coordination number 6 and Cu−N distance of 2.20 A in ammonia, and to number 6 and Cu−O distance of 2.20 A in water. The RDFs, coordination number distributions, and pair interaction energy distribution analyses indicate that ligand exchange reactions take place more easily in water than in liquid ammonia.

The Cu2+−NH3 pair potential and the corresponding three-body correction functions have been newly constructed on the basis of ab initio calculations of double-ζ quality. Monte Carlo simulation resu...

The structures, gas-phase acidities and vibrational spectra of selenoformic O-acid, selenoformic Se-acid and several of their derivatives RCSeOH and RCOSeH (R = H, F, Cl, NH2, CH3) were investigated by DFT calculations. Geometry optimizations and frequency computations were performed at the Becke3LYP level of theory with the 6-311+G(d,p) basis set. For all 10 acids studied, the syn conformers are predicted to have the lowest energy. The syn−anti enthalpy difference varies between 0.3 and 9.9 kcal mol-1, the syn selenol acids being substantially more stable than their syn selenoxo counterparts. The substitution of oxygen by selenium in the selenocarboxylic acids studied leads to increased acidity. The selenol acids are weaker acids than the selenoxo derivatives.

The description of nonadditive contributions in the first hydration shell of Na+ and K+ has been improved by performing molecular dynamics simulations based on combined ab initio quantum mechanical...

Ab initio molecular orbital calculations at the CBS-Q level of theory have been performed for 72 complexes in order to determine the effects of molecular structure on gas-phase basicity toward H+, Li+, and Mg2+. It is shown that basicities toward metal cations Li+ and Mg2+ differ considerably from the corresponding gas-phase basicities toward H+. Calculated interaction energies vary as H+ ≫ Mg2+ > Li+. The relative basicities of the bases studied depend characteristically on type of cation and coordination site.

Reaction rate constants for the concerted proton transfer in cyclic hydrogen fluoride clusters obtained by using a variational transition state theory approach with high-level data at three stationary points and with interpolated corrections based on semiempirical Hamiltonians specially parametrized for these systemsMP2/6-311++G(3df,3pd)///PM3-SRPare reported. For the first time multidimensional tunneling is included in the large curvature limit. The corresponding small curvature limit is benchmarked against higher level calculations. The ridge of the reaction swath is defined in a general way and serves together with an estimate of the barrier width as a validation of the potential energy hypersurfaces. Recent experimental and theoretical results concerning carboxylic acids, whose hypersurfaces are found to be very similar to those of the HF clusters, underline the relevancy of the investigated concerted proton exchange motion. A reaction rate constant k(300 K) = 2.86 × 109 s-1 is found for (HF)5, which ...

A detailed analysis of reaction-path dynamics of hydrogen abstraction from ethane, fluoroethane, and chloroethane by hydroxyl radical has been performed by the variational transition-state theory augmented with multidimensional semiclassical tunneling approximations. The minimum energy path and its first and second derivatives were calculated at the MP2(full)/6-31G(d,p) level of theory. The calculated barrier heights were further improved by Gaussian-2(MP2) methodology. This dual-level dynamic approach has been used to calculate the reaction rate constants for temperatures from 200 to 1000 K. The contribution from tunneling effect was evaluated using the semiclassical zero-curvature and small-curvature tunneling approximations. The calculated thermal reaction rate constants agree well with the experimental results. The variational effects on the location of central dynamical bottleneck are significant for all three reactions. The competition between potential and vibrational energies in determining the lo...

The concerted proton transfer hypersurface of cyclic water and hydrogen fluoride clusters has been described by high-level molecular quantum mechanical calculations. For the cyclic water clusters the concerted proton transfer transition states have been investigated for the first time with methods including treatment of dynamic electron correlation. The crucial importance of dynamic electron correlation for the barrier heights is demonstrated, A detailed analysis of the minimum energy path has been performed. The reaction swath of the concerted proton transfer was examined indicating a reasonable description by harmonic approximation of the energy hypersurface. Transfer rates have been calculated by means of variational transition state theory with interpolated corrections (VTST-IC) and dual-level direct dynamics (DLDD), both with semiclassical tunneling corrections. Tunneling is very efficient in the concerted proton exchange reaction of the cyclic hydrogen-bonded clusters under investigation. Rate constants for the concerted exchange of hydrogens in important hydrogen fluoride vapor phase species are reported for the first time. In the hydrogen fluoride tetramer and pentamer the concerted proton exchange of four and five protons, respectively, takes place with reaction rates that an comparable with the concerted exchange rates in carboxylic acid dimers and is not hindered by the large number of simultanously moving protons. The concerted proton exchange rates in the studied water clusters are comparably low because of higher exchange barriers. It is shown that hydrogen fluoride clusters can be used to a large extent as ''simplified'' experimental and theoretical models for water clusters.

In order to describe the zinc−water interaction correctly, a new ab initio potential was developed consisting of pair interaction terms as well as three-body contributions. Within this approach, it was possible to correct for the well-known failures of pair potentials in describing solvation phenomena of such ions. A first-shell coordination number of 6 in agreement with experimental data was obtained from Monte Carlo simulations of a single zinc(II) ion in water. The structure of the hydrated ion is discussed in terms of radial density functions and coordination number, energy, and angular distributions.

Optimal nonlinear transformations provided by alternating conditional expectations (ACE) method are used to study the relationship between electronic structure and antimalarial activity of mefloquine and its substituted derivatives. The electronic structure of these molecules is featured by atomic net charges evaluated on the basis of CNDO/2 molecular orbital calculations. A comparison to multiple linear regression (MLR) and partial least squares (PLS) method is also included. The results show that ACE can be a useful and well-suited technique for establishing QSAR and predicting the pharmacological activity of compounds.

Monte Carlo simulations have been performed for the system 0.5 M CuCl 2 /5 M NaCl/H 2 O which has been round to induce condensation or amino acids to peptides. Two water potentials (CF and MCY) have been used and compared. Methodical aspects and comparison with experimental data are in favor or the results obtained with the CF potential. A large number or solvate species are round to coexist in the system, and the results or the simulations allow one to conclude which species should be most active in the reaction mechanism or the peptide formation

Structure and dynamics investigations of Au(I) ion in liquid ammonia have been performed by means of a molecular dynamics simulation based on ab initio quantum mechanical/molecular mechanical forces, where the first solvation shell was treated by quantum mechanics at Hartree-Fock level. The outer region of the system was described using a newly constructed classical three-body corrected potential. A rigid structure of the first solvation shell was observed with an average Au-N distance of 2.15 A and a coordination number of 2.0.